1. Field of the Invention
This invention relates to a modulation device which modifies an input signal in order to compensate for nonlinear input/output characteristics of an amplifier in a modulation system using an amplitude and a phase of a carrier as information such as a QPSK (quaternary phase shift keying) modulation system.
2. Description of the Prior Art
A recent trend in communication system design has been to narrow the effective frequency band of a channel in order to attempt effective utilization of the available frequency spectrum in radio communication. When a channel band width becomes narrow, deterioration in a transmission signal caused by nonlinearity of a transmitter amplifier becomes a problem. The reason is that intermodulation components of odd orders such as the third order, the fifth order, and the like are generated by the nonlinear input/output characteristic of an amplifier (AM/AM conversion) and drifting of an output signal phase as an input signal amplitude increases (AM/PM conversion), and consequently interference with adjacent channels is easily generated.
As a conventional device for correcting a nonlinear characteristic of an amplifier and correcting changes in the amplifier characteristic caused by temperature changes and the like, there is known a modulation device shown in Japanese Patent Disclosure Publication No. 214843/1986. FIG. 1 is a circuit diagram showing such a conventional modulation device for a quadrature transmission system in which two carriers in phase quadrature are transmitted simultaneously. In FIG. 1, reference numerals 1 and 2 each are an input terminal, reference numeral 3 is an output terminal to an amplifier, 4 is an input terminal into which part of an output of the amplifier is inputted, 10 is a random access memory (RAM) in which data can be rewritten, 20 is a quadrature modulator, 30 is a quadrature demodulator, 40 is an oscillator, 50 is a D-A (digital-analog) converter, 60 is an A-D (analog-digital) converter, 70 is a subtractor circuit, 80 is a modification value generating circuit, and 90 is an adder circuit.
Signals 1-I and 1-Q applied to the input terminals 1 and 2 represent a real part and an imaginary part of a signal series obtained by sample-quantizing a complex signal S(t)=I sin .omega..sub.c t+Q cos .omega..sub.c t. Modified data for compensating the nonlinearity of the amplifier are stored in the RAM 10, which outputs signals 2-I and 2-Q, corresponding to the signals 1-I and 1-Q, for which the nonlinearity of the amplifier is taken into account. The signals 2-I and 2-Q are converted into analog signals by the D-A converter 50, and modulated in the quadrature modulator 20. On the other hand, part of the output of the amplifier (not shown) which is inputted to the input terminal 4 is demodulated by quadrature demodulator 30, converted into a digital signal by the A-D converter 60, and then subtracted from the data of the input signals 1-I and 1-Q by the subtractor circuit 70. If the nonlinearity of the amplifier has been correctly compensated, the output of the subtractor circuit 70 will be 0. But, in the case where the characteristic of the amplifier changes due to temperature changes and the like, the output of the subtractor circuit 70 will not be 0, and at that time, outputs of the modification value generating circuit 80 are added to the values outputted by the RAM 10, and then the memory location in RAM 10 corresponding to signals 1-I and 1-Q as an address is rewritten by the added values.
In this way, in a conventional modulation device shown in FIG. 1, even in the case where the characteristic of the amplifier varies due to temperature changes and the like, distortions generated in the amplifier are compensated.
However, in such a conventional modulation device as shown in FIG. 1, since the modification for compensation of distortions is carried out for both of the input signals 1-I and 1-Q, a very large capacity of the RAM 10 is needed, for example on the order of 100MB, and there are problems that the modulation device becomes large-sized, its power consumption is large, its cost is high and its efficiency is low.